Process of providing oxidizable refractory-metal bodies with a corrosion-resistant surface coating



PRGCESS 6F PRUVEDHNG OXIDIZABLE REFRAC- TQRY-METAL BUDDIES WKTH A CORRGSION- RESlSTANT SURFACE CQATING Richard Kieifer and Karl Sedlatschelt, Reutte, Tirol,

Austria, assignors to Schwarzkopf Development Corporation, New York, N.Y., a corporation of Maryland No Drawing. Filed Feb. 2, 1960, Ser. No. 6,114 Claims priority, application Austria dune 4, 1953 12 Qlairns. (Cl. 1l7--1tl2) This application is a continuation-in-part of our copending applications Serial Nos. 817,433 and 817,434, both filed May 25, 1959, both now abandoned, and the contents of the aforesaid two applications are repeated and made parts of the present application.

This invention relates to the protection of molybdenum and also of tungsten, tantalum, niobium, and their alloys, against oxidation and corrosion when exposed at high temperatures to oxidizing and/ or corroding media. Molybdenum, which is available in great abundance, has high hot-strength and would be of great value in applications requiring a strong metal body which will retain its shape at elevated temperatures such at 700 C. and higher. Among the applications for which shaped refractory bodies of molybdenum and/or of the other foregoing refractory metals and their alloys, are of value, are heater rods, gas turbine buckets, rocket nozzles, combustion chambers, and like other critical applications.

However, molybdenum and also the refractory metals of tungsten, tantalum, niobium, and their alloys, readily oxidize when exposed to oxidizing atmospheres at elevated temperatures. For example, molybdenum oxides to form a trioxide which sublimes at 795 C. Therefore, in spite of their high refractoriness and hot-strength, a body of molybdenum, tungsten, tantalum or niobium, will soon deteriorate and corrode when operating at high temperatures under oxidizing temperatures. Accordingly,

the exterior of a shaped molybdenum body has to be provided with a corrosion and oxidation resistant protective coating layer when it is exposed to oxidizing and/ or corroding atmospheres or'media at elevated temperatures.

Although many attempts have been made in the past to provide such readily oxidizable, shaped refractorymetal bodies with a corrosion-resistant coating that would provide the desired protection against corrosion and oxidation at high temperatures Within an oxidizing medium, none, of them proved successful. The present invention is based on the discovery that a very effective corrosionresistant alloyed coating layer for a shaped metal body of molybdenum or other refractory metals referred to, may be formed thereon by treating the exterior surfaces of such shaped bodies with molten copper containing dissolved therein an alloying metal of either silicon or chromium, for causing the alloying metal content thereof to form with the molybdenum of the exposed surface of the treated refractory-metal body, a tightly adhering, dense alloyed coating layer of silicized or chromized molybdenum, respectively, until a continuous, dense coating of such molybdenum-silicon or molybdenum-chromium alloy has been formed on the entire exterior, exposed surface of the shaped refractory-metal body, and after cooling the so-silicized or chromized refractorymetal body, the remaining copper content of the soformed alloyed coating layer together with unused alloying metal contained in the remaining copper, is removed by applying to the so-coated exterior of the body a molten solvent metal which is bare of or free of silicon or chromium, respectively, until all remaining copper of the silicized or chromized body coating layer together 3,936,88h Patented Apr. 23, 1963 "ice with the alloying metal contained therein are dissolved in the so-applied molten solvent metal.

After first cleaning it, a shaped molybdenum body, for example a molybdenum heater rod, which is to be exposed at elevated temperatures above 700 C. to an oxidizing medium, is held immersed in molten copper containing dissolved therein 3% to 30% of silicon until the silicon of the molten copper combines with the molybdenum on the exposed exterior surface of such molybdenum body into a tightly adhering, continuous, dense coating layer of molybdenum-silicide of uniform composition and thickness. After cooling the so-silicized molybdenum body, the remaining copper adhering to the molybdenum-silicide coating thereof, together with the silicon content of the remaining copper, are removed by applying to or dipping the so-coated molybdenum body in molten solvent metal, such as zinc which issubstantially bare of or free of silicon, until all copper on the exterior of the molybdenum-silicide coating has been dissolved in such applied silicon-free molten solvent metal. The so-produced molybdenum-silicide coating adheres tightly to the molybdenum body, is of uniform composition and thickness, and provides the coated molybdenum body with a higher order of protection against corrosion than obtainable with molybdenum and silicide coatings produced by heretofore known processes. Superior results are obtained by applying to the molybdenum body a coating of molten copper having dissolved therein 10% to 15% silicon. Copper is a very good solvent for silicon in proportions up to 30%. However, molybdenum is not dissolved in molten copper.

Similar superior silicide coatings may be formed in a similar manner on bodies of tungsten, tantalum, niobium and their alloys, and on molybdenum alloys, by applying to their exterior surfaces a coating layer of molten copper having dissolved therein 3% to 30% silicon, and thereafter treating the body in the same Way as the molybdenum body, for forming on its exterior, a protective silicide coating layer. niobium, and their alloys, are not dissolved, or dissolved only to a negligible extent, in molten copper.

In practicing the invention, good results are obtained by dipping the molybdenum body in a bath of molten copper having the silicon dissolved therein. Alternatively, molten copper having the silicon dissolved therein, is applied to the exterior surface of the molybdenum body by spraying, evaporating, and subjecting the so-coated body to heat treatments wherein the silicon of the applied coating combines with molybdenum of hte underlying body into molybdenum-silicide. Alternatively, copper powder having admixed thereto the specified proportion of silicon, is appliedas a thin powder layer to the exterior of the molybdenum body, and the so-coated molybdenum body is heated to melt the applied powder layer and to cause the silicon to combine with the molybdenum of the treated body into the desired molybdenumsilicide coating layer.

When dipping the molybdenum body in molten siliconcontaining copper, the silicon of the molten copper reacts with the molybdenum which is at the temperature of the molten copper bath, to form with the molybdenum of the body the coating layer of molybdenum-silicide, such as molybdenum-disilicide. After-removing the molybdenum body on which the coating layer of molybdenum-silicide was so formed, the excess of molten copper adhering to the exterior of the coating is drained and the so-coated molybdenum body is cooled. The so-coated, cooled molybdenum body has a tightly adhering exterior surface layer of the desired molybdenum-silicide, which is of uniform composition and thickness over the entire exterior surface of the dipped molybdenum body. The excess Tungsten, tantalum,

copper adhering to the molybdenum-silicide coating may be removed by treating the exposed surface of the socoated molybdenum body with a suitable agent such as very diluted nitric acid solution, which dissolves the adhering copper together with the unused silicon contained therein. The nitric acid treatment to remove the adhering copper has no harmful effect on the previously formed uniform molybdenum-silicide layer.

With the process of the invention, it is possible to form on the exposed surfaces of a molybdenum body, a molybdenum-silicide layer of uniform thickness which tightly adheres to the underlying molybdenum body, with molten copper containing 3% to 30% silicon, and maintained at a temperature in the range of 800 C. to 1300 C. No reaction takes place between the molybdenum of the sotreated body and the molten copper with which it remains in contact during the treatment. Very effective and uniform molybdenum-silicide coatings are obtained with the silicon-containing molten copper held in contact with the surface of the molybdenum body for forty minutes. Also, satisfactory coatings may be obtained with tenminute treatments. Best results are obtained with molten copper containing to silicon.

The silicide-coating-forming molten copper may contain in addition to the silicon content thereof, up to 50% of the copper content thereof, metal additions consisting of tin (Sn), zinc (Zn), aluminum (Al) or silver (Ag). The silicon-free molten solvent metal may consist of copper, zinc, or tin, or their alloys. Such silicon-free molten solvent metal may contain additions of bismuth, cadmium, lead, or their alloys, up to 50% of their content. Unless otherwise specified, throughout the specification and claims, all proportions are given by weight.

As another example, a shaped molybdenum body which is to be exposed at elevated temperatures above 700 C. in an oxidizing medium, is dipped in molten copper containing dissolved therein 0.5% or preferably 2% to 10% chromium, for about two or more hours, or until the chromium of the molten copper diffuses into and alloys with the underlying exterior surface of the metal body into a tightly-adhering, continuous, dense, chromized molybdenum layer, of uniform composition and thickness.

With such treatment, it is possible to obtain a chromized coating layer having a thickness of 30 to 100 microns. By prolonging the treatment, chromized layers of greater thickness maybe obtained. As long as the entire exterior of the treated molybdenum body is held under a cover of chromium-containing molten copper, the chromizing treatment may be carried on in an air atmosphere, since the chromium-containing molten copper protects the treated surface against oxidation. After removing the so-chromized, shaped molybdenum body from the chromium-containing molten copper, the remaining solidified copper adhering to the exterior chromized molybdenum coating, together with the unused chromium content of such remaining copper, is removed by applying to or dipping the so-coated molybdenum body in molten solvent-.metal which is substantially bare of or free of chromium, until all copper remaining on the chromized molybdenum coating layer has been dissolved in such applied chromiumfree molten solvent metal. metal may consist of copper, zinc, tin, or their alloys. Such chromium-free molten solvent metal may also contain metal additions of bismuth, cadmium, lead, or their alloys, up to 50% of its content.

If acids, such as nitric acid, or alkaline solvents are used for removing excess copper remaining in or adhering to the chromized or silicized coating layers of shaped refractory bodies of molybdenum and other refractory metals referred to above, such acid or alkali solvent also attacks the silicized or chromized coating layer of such bodies, thereby impairing the desired protective action of their silicized or chromized coating layers. By using instead, the silicon-free or chromium-free molten metal bath for removing the remaining copper from the previously-formed silicized or chromized coating layer of The chromium-free solvent Example 1 In molten copper that is maintained at 950 C. under air, is dissolved 12% silicon. A molybdenum heater rod of the type used in an electric furnace, is immersed in the molten silicon-containing copper for 30 minutes. The so-treated molybdenum heater rod is removed from the silicon-containing molten copper, and after draining the excess molten copper therefrom, it is cooled to normal temperature. The heater rod is then placed in a bath of silicon-free molten copper and held therein for 30 minutes or until all remaining copper adhering to the molybdenumsilicide coating of the-treated body, together with unused silicon contained therein, are removed from the adhering silicide coating of such shaped molybdenum body. The resulting molybdenum body has a tightly adhering coating layer of molybdenum-silicide about 70 to microns in thickness, of great uniformity in composition and thickness. Such heater rod has a useful life in a furnace at least twenty percent longer than a molybdenum heater rod having a silicide coating made by known prior processes. The thickness of the coating may be increased, for instance to microns, or decreased to 60 microns, by lengthening or shortening the time during which the silicon-containing molten copper bath was applied to the surface of the treated molybdenum. body.

Example 2 Example 3 Treatment similar to that described in Example 1, except that the applied molten copper had dissolved therein 15% silver and 12% silicon. This treatment yields a molybdenum body having a uniform, tightly adhering exterior molybdenum-silicide coating similar to that obtained by the treatment of Example 1.

By similar-treatments, bodies of tungsten, tantalum, niobium, and their alloys, and molybdenum alloys, may be provided with similar tightly adhering exterior coating layers of the respective metal silicides, which have a high degree of uniformity in composition and thickness, and which are similarly of marked superiority in resisting corrosion than silicide coatings formed on such metal bodies by known treatments.

Example 4 H at a temperature of 1250 C. A shaped body, such as a heater rod, is immersed in the molten chromium-containing copper, and after bringing it to the temperature of the molten copper bath, the treated body is held in the bath for two hours. The treated body is then removed from the chromium-containing molten copper bath and cooled to normal temperature. The so-treated chromized molybdenum body is then placed in a bath of chromiumfree molten copper and held therein until all remaining copper adhering to the chromized coating of the treated body, together with unused chromium contained therein, are removed from the adhering chromized coating layer of such shaped molybdenum body. The resulting metal body has on its exterior a tightly adhering chromized metal layer 40 microns thick, and which is of uniform composition and thickness. An analysis indicates that the outer, thin chromized coating layer of the metal body contains about chromium.

Example 5 Treatment similar to that described in Example 4, except that the molten copper contains dissolved therein 8% chromium.

Example 6 Treatment similar to that described in Example 4, except that the molten copper in which the metal body is immersed, contains dissolved therein 4% chromium.

By similar treatments, shaped bodies of molybdenum, tungsten, tantalum, niobium, and their alloys, may be provided with a chromized corrosion-resistant exterior coating layer.

The temperature at which the chromizing treatment with the chromium-containing molten copper has to be carried on, may be reduced by substituting for up to 50% of the copper content, one of the metals of the group consisting of tin, silver, zinc, cadmium, aluminum and lead. The molten chromium-containing copper may also contain silicon dissolved therein, in which case the 2% to 10% of the dissolved chromium is replaced by up to 35% silicon.

The chromized coating layer of a metal body produced in accordance with the invention, may have further diffused therein aluminum or beryllium, by known diffusion treatments, to provide it with an outer chromized metal stratum containing diffused aluminum or beryllium.

The features and principles underlying the invention described above in connection with specific exemplifications thereof, will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claims shall not be limited to any specific features or details described in connection with the exemplifications thereof.

We claim:

1. In the process of producing on a shaped refractory metal body consisting essentially of a refractory metal selected from the group consisting of Mo, W, Ta, Nb and their alloys, and an exterior, corrosion-resistant alloyed coating layer of substantially uniform density and thickness adhering to the exposed exterior surface of such shaped body, which coating layer consists essentially of an alloy of said refractory metal with an alloying metal selected from the group consisting of silicon and chromium, the procedure of subjecting the exposed body surface of said shaped refractory metal body to molten copper containing dissolved therein 0.5% to 30% of said alloying metal until the alloying metal of the applied molten copper combines with the underlying refractory metal of said body into a continuous alloyed coating layer tightly adhering to the underlying shaped body, and consisting of said refractory metal and of said alloying metal, thereafter cooling the so-treated body, thereafter applying to the exposed surface of the so-formed alloyed coating layer a molten solvent metal which is free of said alloying metal and is selected from the group consisting of copper, zinc, tin and their alloys, and continuing said application of said molten solvent metal until all copper adhering to said so-formed alloyed coating layer together with any alloying metal content contained in said adhering copper is removed from said so-forrned alloyed coating layer.

2. In the process of producing an exterior, corrosionresistant alloyed coating layer on a shaped refractory metal body as claimed in claim 1, the molten metalconta-ining dissolved therein said alloying metal also containing up to 50% of its copper content by weight an addition metal selected from Sn, Zn, Al and Ag and their alloys.

3. In the process of producing an exterior, corrosionresistant alloyed coating layer on a shaped refractory metal body as claimed in claim 1, the alloying metalfree solvent metal containing up to 50% of its content by weight of an addition metal selected from the group consisting of aluminum, bismuth, cadmium, lead and their alloys.

4. In the process of producing an exterior, corrosionresistant alloyed coating layer on a shaped refractory metal body as claimed in claim 1, the molten metal containing dissolved therein said alloying metal also containing up to 50% of its copper content by weight an addition metal selected from Sn, Zn, Al and Ag and their alloys, the alloying metal-free solvent metal containing up to 50% of its content by weight of an addition metal selected from the group consisting of aluminum, bismuth, cadmium, lead and their alloys.

5; In the process of producing on a shaped refractory metal body consisting essentially of a refractory metal selected from the group consisting of Mo, W, Ta, Nb and their alloys, an exterior, corrosion-resistant alloyed coating layer of substantially uniform density and thick ness adhering to the exposed exterior surface of such shaped body, which coating layer consists essentially of an alloy of said refractory metal with an alloying metal consisting essentially of silicon, the procedure of subjecting the exposed body surface of said shaped refractory metal body to molten copper containing dissolved therein 3% to 30% of silicon until the alloying metal of the applied molten copper combines with the underlying refractory metal of said body into a continuous alloyed coating layer tightly adhering to the underlying shaped body, and consisting of said refractory metal and of said alloying metal, thereafter cooling the so-treated body, thereafter applying to the exposed surface of the so-formed alloyed coating layer a molten solvent metal which is free of said alloying metal and is selected from the group consisting of copper, zinc, tin, and their alloys, and continuing said application of said molten solvent metal until all copper adhering to said so-formed alloyed coating layer together with any alloying metal content contained in said adhering copper is removed from said so-formed alloyed coating layer.

6. In the process of producing an exterior, corrosionresistant alloyed coating layer on a shaped refractory metal body as claimed in claim 5, the molten metal containing dissolved therein said alloying metal also containing up to 50% of its copper content by weight an addition metal selected from Sn, Zn, Al and Ag and their alloys.

7. In the process of producing an exterior, corrosionresistant alloyed coating layer on a shaped refractory metal body as claimed in claim 5, the silicon-free solvent metal containing up to 50% of its content by weight of an addition metal selected from the group consisting of aluminum, bismuth, cadmium, lead and their alloys.

8. In the process of producing an exterior, corrosionresistant alloyed coating layer on a shaped refractory metal body as claimed in claim 5, the molten metal containing dissolved therein said alloying metal also containing up to 50% of its copper content by weight an addition metal selected from Sn, Zn, A1 and Ag and their alloys, the silicon-free solvent metal containing up to 50% of its content by weight of an addition metal selected from the group consisting of aluminum, bismuth, cadmium, lead and their alloys.

9. In the process of producing on a shaped refractory metal body consisting essentially of a refractory metal selected from the group consisting of Mo, W, Ta, Nb and their alloys, an exterior, corrosion-resistant alloyed coating layer of substantially uniform density and thickness adhering to the exposed exterior surface of such shaped body, which coating layer consists essentially of an alloy of said refractory metal with an alloying metal consisting essentially of chromium, the procedure of subjecting the exposed body surface of said shaped refractory metal body to molten copper containing dis solved therein 0.5% to 25% of chromium until the alloying metal of the applied molten copper combines With the underlying refractory metal of said body into a continuous alloyed coating layer tightly adhering to the underlying shaped body, and consisting of said refractory metal and of said alloying metal, thereafter cooling the so-treated body, thereafter applying to the exposed surface of the so-formed alloyed coating layer a molten solvent metal which is free of said alloying metal and is selected from the group consisting of copper, Zinc, tin and their alloys, and continuing said application of said molten solvent metal until all copper adhering to said so-formed alloyed coating layer together with any alloying metal content contained in said adhering copper is removed from said so-formed alloyed coating layer.

10. In the process of producing an exterior, corrosionresistant alloyed coating layer on a shaped refractory metal body as claimed in claim 9, the molten metal containing dissolved therein said alloying metal also containing up to 50% of its copper content by Weight an addition metal selected from Sn, Zn, Al and Ag and their al loys.

11. In the process of producing an exterior, corrosionresistant alloyed coating layer on a shaped refractory metal body as claimedin claim 9, the chromium free solvent metal containing up to 50% of its content by Weight of an addition metal selected from the group consisting of aluminum, bismuth, cadmium, lead and their alloys.

12. In the process of producing an exterior, corrosionresistant alloyed coating layer on a shaped refractory metal body as claimed in claim 9, the molten metal containing dissolved the-rein said alloying metal also containing up to 50% of its copper content by Weight an addition metal selected from Sn, Zn, Al and Ag and their alloys, the chromium-free solvent metal containing up to 50% of its content by weight of an addition metal selected from the group consisting of aluminum, bismuth, cadmium, lead and their alloys.

References Cited in the file of this patent UNITED STATES PATENTS 2,763,921 Turner et a1 Sept. 25, 1956 2,788,289 Deuble Apr. 9, 1957 2,788,290 Deuble Apr. 9, 1957 2,848,352 Noland et al. Aug. 19, 1958 2,930,106 WrotnoWski Mar. 29, 1960 FOREIGN PATENTS 463,258 Great Britain Mar. 22, 1937 

1. IN THE PROCESS OF PRODUCING ON A SHAPED REFRACTORY METAL BODY CONSISTING ESSENTIALLY OF A REFRACTORY METAL SELECTED FROM THE GROUP CONSISTING OF MO, W, TA, NB AND THEIR ALLOYS, AND AN EXTERIOR, CORROSION-RESISTANT ALLOYED COATING LAYER OF SUBSTANTIALLY UNIFORM DENSITY AND THICKNESS ADHERING TO THE EXPOSED EXTERIOR SURFACE OF SUCH SHAPED BODY, WHICH COATING LAYER CONSISTS ESSENTIALLY OF AN ALLOY OF SAID REFRECTORY METAL WITH AN ALLOYING METAL SELECTED FROM THE GROUP CONSISTING OF SILICON AND CHROMIUM, THE PROCEDURE OF SUBJECTING THE EXPOSED BODY SURFACE OF SAID SHAPED REFRACTORY METAL BODY TO MOLTEN COPPER CONTAINING DISSOLVED THEREIN 0.5% TO 30% OF SAID ALLOYING METAL UNTIL THE ALLOYING METAL OF THE APPLIED MOLTEN COPPER COMBINES WITH THE UNDERLYING REFRACTORY METAL OF SAID BODY INTO A CONTINUOUS ALLOYED COATING LAYER TIGHTLY ADHERING TO THE UNDERLYING SHAPED BODY, AND CONSISTING OF SAID REFRACTORY METAL AND OF SAID ALLOYING METAL, THEREAFTER COOLING THE SO-TREATED BODY, THEREAFTER APPLYING TO THE EXPOSED SURFACE OF THE SO-FORMED ALLOYED COATING LAYER A MOLTEN SOLVENT METAL WHICH IS FREE OF SAID ALLOYING METAL AND IS SELECTED FROM THE GROUP CONSISTING OF COPPER, ZINC, TIN AND THEIR ALLOYS, AND CONTINUING SAID APPLICATION OF SAID MOLTEN SOLVENT METAL UNTIL ALL COPPER ADHERING TO SAID SO-FORMED ALLOYED COATING LAYER TOGETHER WITH ANY ALLOYING METAL CONTENT CONTAINED IN SAID ADHERING COPPER IS REMOVED FROM SAID SO-FORMED ALLOYED COATING LAYER. 